1. Field of the Invention
The present invention relates to an object, a method, or a manufacturing method. The present invention relates to a semiconductor device, a display device, a light-emitting device, or a manufacturing method thereof, for example. In particular, the present invention relates to a semiconductor device including an oxide semiconductor, a display device including an oxide semiconductor, or a light-emitting device including an oxide semiconductor, for example. Alternatively, the present invention relates to a display device for displaying an image.
2. Description of the Related Art
A variety of portable electronic devices, such as a cell phone, a smartphone, a personal computer, a tablet terminal, a portable game machine, and a portable music player, have recently come into widespread use.
In the display portion, a light-emitting device including a light-emitting element, a liquid crystal display device, an electronic paper performing display by an electrophoretic method or an electronic liquid powder method, or the like can be typically used.
For example, research and development of light-emitting elements using electroluminescence (EL) have been actively carried out. In a basic structure of such a light-emitting element, a layer containing a substance with a light-emitting property is interposed between a pair of electrodes. By applying a voltage to this element, light emission can be obtained from the light-emitting substance.
Since the above light-emitting element is a self-luminous type, a light-emitting device using this light-emitting element has advantages such as high visibility, no necessity of a backlight, low power consumption, and the like. Further, such a light-emitting element also has advantages in that the element can be formed to be thin and lightweight and that response time is high.
In order to improve flexibility and impact resistance in addition to its thinness and lightness, a flexible substrate has been proposed to be used in a light-emitting device including the light-emitting element.
Further, as a transistor for driving a display element of a display device, a transistor using a semiconductor thin film which is formed over a substrate having an insulating surface is widely used. A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to such a transistor. In recent years, an oxide semiconductor has attracted attention.
Examples of such a transistor include a transistor in which zinc oxide (ZnO) is used as an oxide semiconductor and a transistor in which InGaO3(ZnO)m is used as an oxide semiconductor. A technique in which a switching element, such as a transistor, in which a channel formation region is formed in an oxide semiconductor film, is used for manufacturing an active matrix display device has also been developed.
For example, in Patent Document 1 and Patent Document 2, a display device in which a transistor including an oxide semiconductor in a channel formation region is used for controlling a display element is described. The transistor including the oxide semiconductor in the channel formation region has many advantages in that the transistor has higher field-effect mobility than a transistor using amorphous silicon which is widely spread, can be manufactured with a higher throughput and at lower cost than a transistor using polycrystalline silicon, has extremely low off-state leakage current, and has excellent switching characteristics.